Method for electrochemical reduction of co2 in an electrochemical cell

ABSTRACT

The invention refers to method for electrochemical reduction of CO 2  in an electrochemical cell having a cathode and an anode. The method comprises: at the cathode, reducing the CO 2 ; at the anode, oxidizing a first hydrocarbon generating a second hydrocarbon.

TECHNICAL FIELD

The present invention relates to a method for electrochemical reductionof CO₂ in an electrochemical cell.

BACKGROUND

Power plants for electric power generation produce large amounts ofcarbon dioxide by combustion of a fuel. Currently it is envisaged theneed of a reduction of the carbon dioxide emission, because of theenvironmental impact of the carbon dioxide. In order to limit carbondioxide emissions, power plants can be provided with separation plantswhich remove the carbon dioxide from the flue gas generated by the powerplant; a gas with a low content of carbon dioxide is thus vented intothe atmosphere while the carbon dioxide can be sequestered or used inanother way.

In order to use the large amount of carbon dioxide collected by aseparation plant associated to a power plant, US 2014/0151240 proposesto convert the carbon dioxide in a useful product in an electrochemicalcell. In particular, US 2014/0151240 discloses to provide an absorberdownstream of the power plant. At the absorber a capture solutioncaptures the carbon dioxide and is then forwarded to the cathode of theelectrolytic cell where the carbon dioxide is reduced to a hydrocarbon.According to US 2014/0151240 H₂O is dissociated at the anode accordingto the equilibrium

H₂O

½O₂+2H⁺+2e ⁻

having a ΔG=+948.4 kJ/mole.

Since the ΔG (i.e. variation of Gibbs energy) is positive, energy (inthe form of electric energy) has to be supplied for the reaction tooccur and since the value of ΔG is high, the amount of energy to besupplied is high.

U.S. Pat. No. 8,444,844 proposes carbon dioxide reduction with anelectrochemical cell in which at the anode an oxidation product can bemade that can be subsequently used in producing another carbon compound.In particular, U.S. Pat. No. 8,444,844 teaches to supply, at thecathode, carbon dioxide together with a salt, such as bromide salt, and,at the anode, a reactant such as an halide salt.

According to U.S. Pat. No. 8,444,844 additional reactors are needed forthe products discharged from the electrolytic cell to be converted intohydrocarbon. This implies plant complexity and costs.

US 2013/0140187 proposes carbon dioxide reduction with anelectrochemical cell in which the cathode is supplied with carbondioxide and the anode with an alcohol; at the cathode a first productsuch as CO, formic acid, formaldehyde, methanol oxalate, etc. arecollected, and at the anode a second product such as aldehyde, a ketone,carboxylic acid, etc. are collected.

SUMMARY

An aspect of the invention includes providing a method forelectrochemical reduction of CO₂ in an electrochemical cell and anelectrochemical cell that requires limited electric energy supply whencompared to the energy needed with H₂O dissociation and, in addition,allow production of a hydrocarbon.

These and further aspects are attained by providing a method and anelectrochemical cell in accordance with the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages will be more apparent from thedescription of a preferred but non-exclusive embodiment of the methodand electrochemical cell, illustrated by way of non-limiting example inthe accompanying drawings, in which:

FIG. 1 shows an electrochemical cell in an embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following the electrochemical cell is described first. Theelectrochemical cell can have the features of the electrochemical celldescribed in US 2014/0 151 240. The electrochemical cell 1 comprises abody 2 with a membrane 3 defining a cathode chamber 4 housing a cathode5 and an anode chamber 6 housing an anode 7.

The cathode chamber 4 houses a catholyte 8 surrounding the cathode 5,and the anode chamber 6 houses an anolyte 9 surrounding the anode 7.

The cathode 5 and/or the anode 7 comprise an electro conductive bar, forexample comprising a metal bar or a bar with a metal coating; inaddition the bar defining the cathode and/or anode can be a solid bar ora mesh bar.

The membrane 3 is an ion exchange membrane, i.e. a membrane that permitsexchange of ions between the cathode and anode; for example the membranecan have the features described in US 2014/0 151 240.

The catholyte 8 and the anolyte 9 are preferably liquid catholyte andliquid anolyte and can be the same as those described in US 2014/0 151240. Alternatively, the electrolytic cell can be a solid oxideelectrolytic cell (with solid catholyte and anolyte) in which gaseoushydrocarbons are used for the reactions at the anode.

In addition, in order to improve contacting a water insoluble organiccompound like the anolyte with the anode 7, a wetting agent such asanionic, cationic, non-ionic or amphoteric surfactants can be containedin the anolyte 9 and/or a hydrophobic coating like hydrocarbons orfluorinated hydrocarbons can be provided on the anode 7.

The electrochemical cell 1 also has an electric energy feeder 11. Thisfeeder 11 is usually a DC electric energy feeder and can for exampleinclude a battery and/or rectifier connected to the electric grid and/oranother device. The feeder 11 is connected between the cathode 5 and theanode 7; as known in the art, the cathode 5 is connected to the positivepole and the anode is connected to the negative pole of the feeder 11.

The electrochemical cell 1 preferably also has a circulating path 12 forcirculating the catholyte outside of the cathode chamber 4 and a CO₂feeder 13 into the circulating path 12 for the catholyte.

Likewise, the electrochemical cell 1 also has a circulating path 15 forcirculating the anolyte outside of the anode chamber 6 and a feeder 16for supplying a first hydrocarbon into the circulating path 15 for theanolyte.

The electrochemical cell 1 further has outlets 18, 19 respectively atthe cathode chamber 4 and at the anode chamber 6 for collecting products(typically gas products) generated during operation.

The operation of the electrochemical cell is apparent from thatdescribed and illustrated and is substantially the following.

Carbon dioxide is supplied via the CO₂ feeder 13 into the circulatingpath 12 and is supplied, together with the circulating catholyte intothe cathode chamber 4. Catholyte circulation allows mixing of thecatholyte and improving ion transport and the reactions at the cathode5.

In addition, a first hydrocarbon is supplied into the circulating path15; for example ethane C₂H₆ can be supplied into the circulating path15. Other hydrocarbons can be used, such as for example propane C₃H₈,coal, lignite, coke, etc. In addition, instead of one hydrocarbon alsomixtures of hydrocarbons can be used; these mixtures can comprise any ofethane C₂H₆, propane C₃H₈, coal, lignite, coke, or other hydrocarbons.The hydrocarbon is mixed with the anolyte and hydrocarbon and anolyteare supplied together into the anode chamber 6. Anolyte circulationallows mixing of the anolyte and improving ion transport and thereactions at the anode 7.

Together with carbon dioxide supply into the cathode chamber 4 andhydrocarbon supply into the anode chamber 6, electric energy (typicallyDC electric energy) is supplied to the cathode 5 and anode 7 via thefeeder 11.

Electric energy allows the following electrochemical reactions to takeplace:

at the cathode:

8H⁺+8e ⁻+CO₂

CH₄+2H₂O ΔG=−130.3 kJ/mole

at the anode

4(C₂H₆

C₂H₄+2H⁺+2e ⁻) ΔG=+271.3 kJ/mole

with overall reaction being

4C₂H₆+CO₂

C₂H₄+2H₂O+CH₄

and gathering of methane CH₄ at the cathode 5 and ethylene C₂H₄ at theanode. Naturally even if according to the described example carbondioxide CO₂ is reduced to methane CH₄, in different embodiments CO₂ canbe reduced to CO, formic acid, methanol, ethanol, ethylene, propylene,etc. according to the particular reactions occurring. In addition, atthe cathode 5 also water H₂O and/or some unreacted CO₂ can be collected;these products can be separated downstream of the outlet 18 in aseparator. Likewise, at the anode 7 also water H₂O and/or some unreactedfirst hydrocarbon (e.g. C₂H₆) can be collected; these products can beseparated downstream of the outlet 19 in a separator.

Advantageously, the electrochemical cell, of the invention allowsgathering of a useful product at the cathode and one or morehydrocarbons at the anode (the second hydrocarbon), while consumingcarbon dioxide (at the cathode), with the need of limited electricenergy input (because of the lower ΔG of the reaction at the anode) andwith a substantially simple plant (because no additional reactors tocomplete the reactions and get the hydrocarbons or useful products areneeded).

The present invention also refers to a method for electrochemicalreduction of CO₂ in an electrochemical cell having a cathode and ananode. The method comprises

at the cathode, reducing the CO₂,

at the anode, oxidizing a first hydrocarbon generating a secondhydrocarbon.

The first hydrocarbon comprises at least one among ethane (C₂H₆),propane (C₃H₈), coal, lignite, coke; in a preferred embodiment the firsthydrocarbon is ethane (C₂H₆) and the second hydrocarbon is ethylene(C₂H₄). The first hydrocarbon can be in gas and/or liquid and/or solidform according to e.g. the pressure and temperature and can be a singlecompound or a mixture of different hydrocarbons.

The second hydrocarbon will depend on the first hydrocarbon and theoxidation reaction occurring at the anode 7.

Often, when the hydrocarbon is collected, it contains impurities; themethod thus further comprises removing the second hydrocarbon from theanode and separating the second hydrocarbon from impurities that can forexample comprise the first hydrocarbon.

Naturally the features described may be independently provided from oneanother.

1. A method for electrochemical reduction of CO₂ in an electrochemicalcell having a cathode and an anode, the method comprising at thecathode, reducing the CO₂, at the anode, oxidizing a first hydrocarbongenerating a second hydrocarbon.
 2. The method of claim 1, wherein thefirst hydrocarbon comprises at least one among ethane (C₂H₆), propane(C₃H₈), coal, lignite, coke.
 3. The method of claim 1, wherein the firsthydrocarbon is ethane (C₂H₆) and the second hydrocarbon is ethylene(C₂H₄).
 4. The method of claim 1, wherein the electrochemical cellcomprises a body with a membrane defining a cathode chamber housing thecathode and an anode chamber housing the anode, the cathode chamberhousing a catholyte surrounding the cathode, the method furthercomprising circulating the catholyte outside of the cathode chamber. 5.The method of claim 4, wherein the catholyte is a liquid catholyte. 6.The method of claim 4, wherein the CO₂ is supplied into the catholytecirculating outside of the cathode chamber.
 7. The method of claim 1,wherein the electrochemical cell comprises a body with a membranedefining a cathode chamber housing the cathode and an anode chamberhousing the anode, the anode chamber housing an anolyte surrounding theanode, the method further comprising circulating the anolyte outside ofthe anode chamber.
 8. The method of claim 7, wherein the anolyte is aliquid anolyte.
 9. The method of claim 7, wherein the first hydrocarbonis supplied into the anolyte circulating outside of the anode chamber.10. The method of claim 1, further comprising removing the secondhydrocarbon from the anode and separating the second hydrocarbon fromimpurities.
 11. The method of claim 10, wherein impurities comprise thefirst hydrocarbon.
 12. The method of claim 1, wherein the anode isimmersed in an anolyte containing a wetting agent and/or the anode has ahydrophobic coating.
 13. An electrochemical cell for electrochemicalreduction of CO₂, the electrochemical cell comprising a cathode forreducing the CO₂, an anode for oxidizing a first hydrocarbon generatinga second hydrocarbon.
 14. The electrochemical cell of claim 13, whereinthe first hydrocarbon comprises at least one among ethane (C₂H₆),propane (C₃H₈), coal, lignite, coke.
 15. The electrochemical cell ofclaim 13, further comprising a body with a membrane defining a cathodechamber housing the cathode and an anode chamber housing the anode, thecathode chamber housing a catholyte surrounding the cathode, theelectrochemical cell further comprising a circulating path for thecatholyte outside of the cathode chamber.
 16. The electrochemical cellof claim 15, wherein the catholyte is a liquid catholyte.
 17. Theelectrochemical cell of claim 15, further comprising a CO₂ feeder intothe circulating path for the catholyte.
 18. The electrochemical cell ofclaim 13, wherein the electrochemical cell comprises a body with amembrane defining a cathode chamber housing the cathode and an anodechamber housing the anode, the anode chamber housing an anolytesurrounding the anode, the electrochemical cell further comprising acirculating path for the anolyte outside of the anode chamber.
 19. Theelectrochemical cell of claim 18, wherein the anolyte is a liquidanolyte.
 20. The electrochemical cell of claim 18, further comprising afeeder for the first hydrocarbon into the circulating path for theanolyte.